1. Field of the Invention
This invention relates to signal processing circuits that process signals output from electro-optic probes used for testing of printed-circuit boards of high-speed processing.
This application is based on Patent Application No. Hei 9-307657 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In accordance with tendencies in increases of processing speeds of information communication systems in these days, it is demanded to provide extremely high-speed processing for hardware used for the systems. As probes of sampling oscilloscopes that are required to perform testing of printed-circuit boards of high-speed processing, the recent technology develops high-impedance electro-optic probes (or electric-optic sampling probe, abbreviated by EOS).
FIG. 5 shows an example of simplified construction of an electro-optic probe. Particularly, FIG. 5 shows a head portion H of the electro-optic probe. Herein, an electro-optic crystal 1 is made of electro-optic material such as BSO (i.e., Bi.sub.12 SiO.sub.20). A multilayer dielectric mirror 2 is formed on a lower surface of the electro-optic crystal 1 by vapor deposition, wherein it is capable of reflecting laser beams input thereto. In addition, a metal pin 3 is attached to the lower surface of the electro-optic crystal 1. The aforementioned electro-optic crystal 1 and the metal pin 3 are incorporated in an insulator 4.
When the metal pin 3 of the head portion H is brought into contact with a signal line 6 laid on a board 5, an electric field is caused to occur due to signals transmitted through the wiring, so the electric field is connected with the electro-optic crystal 1. Due to primary electro-optic effect such as Pockel's effect, the electro-optic crystal 1 is brought into a state that a birefringence ratio thereof is varied in response to strength of the "connected" electric field. So, by introducing laser beams into the electro-optic crystal 1 under such a state, the laser beams are changed in polarization states. The laser beams subjected to changes in polarization states are reflected by the multilayer dielectric mirror 2 and are then introduced to a polarization detection optical system (not shown) provided inside of the probe.
In the polarization detection optical system, the laser beam output from the head portion H is split into polarized components, which are perpendicular with each other in orientations, by a polarization beam splitter. Then, the polarized components of the laser beam are respectively converted to electric signals by photodiodes. The electric signals are forwarded to a signal processing circuit.
FIG. 6 is a block diagram showing an example of a configuration of the signal processing circuit. In FIG. 4, a sampling pulse generation circuit 11, which is configured by a fast ramp generation circuit 11a, a slow ramp generation circuit 11b and a comparator 11c as shown in FIG. 7. Herein, the fast ramp generation circuit 11a generates a sawtooth signal FL (see FIG. 8B) based on a trigger pulse signal T (see FIG. 8A), which is supplied thereto from the external and is synchronized with a measured signal. The sawtooth signal FL gradually increases in level with a certain slope from the timing of a trigger pulse and then decreases in level suddenly at the timing that is determined by a width of a display screen of the oscilloscope. The slow ramp generation circuit 11b generates a step-like signal SL (see FIG. 8C) based on the trigger pulse signal T. The step-like signal SL increases in level in a step-like manner, wherein it is increased by a predetermined level at the timing of a trigger pulse. The comparator 11c generates a sampling pulse signal SP (see FIG. 8D) based on the sawtooth signal FL and step-like signal SL. Herein, a sampling pulse is generated at the timing that the sawtooth signal FL coincides with the step-like signal SL in levels. Then, the sampling pulses SP are supplied to an optical pulse generation circuit 12 shown in FIG. 6.
The optical pulse generation circuit 12 uses a semiconductor laser to convert the sampling pulse signal to laser beam pulses (i.e., optical pulses), which are then forwarded to a head portion H (see FIG. 5) of an electro-optic probe DP via an optical fiber amplifier, an optical bandpass filter and a polarization controller, all of which are not shown in FIG. 6. Those optical pulses pass through the electro-optic crystal 1 of the head portion H. Thereafter, they are converted to electric signals by the aforementioned polarization detection optical system of the electro-optic probe DP. The electric signals are input to a receiving light amplification circuit 14.
The receiving light amplification circuit 14 performs differential amplification on output signals of the electro-optic probe DP. Output of the receiving light amplification circuit 14 is forwarded to an analog-to-digital conversion circuit (abbreviated by "A/D conversion circuit") 15. Based on the sampling pulse SP output from the sampling pulse generation circuit 11, the A/D conversion circuit 15 performs sampling with respect to an output of the receiving light amplification circuit 14 which is given at the timing when a prescribed time elapses from the leading-edge timing of the optical pulse. Thus, the A/D conversion circuit 15 converts analog signals, corresponding to results of the sampling, to digital data, which are then forwarded to an image display circuit 16. The image display circuit 16 performs image display processing based on output of the A/D conversion circuit 15.
By the way, the foregoing sampling pulse generation circuit 11 generates one sampling pulse in response to one trigger pulse, which is shown in FIG. 8A to FIG. 8D. For this reason, in the case where a period of the trigger pulse signal is longer than a desired sampling rate, the aforementioned signal processing circuit suffers from a problem that measurement requires an unnecessary long time.